Not Applicable
The present invention is related to the field of battery protection circuits.
The need for special battery protection circuits has increased due to the widespread use of batteries such as lithium-ion batteries, which are inherently less tolerant of adverse operating conditions than are more traditional batteries such as those using nickel-based chemistries. Circuits have been used to detect whether battery voltage or current exceeds a predetermined safe level, and take certain compensatory actions upon detection of such conditions. For example, if excessively high battery current is detected, a protection circuit may turn off a protection transistor arranged in series with the battery, interrupting the flow of current.
For applications in which it is important to make efficient use of stored battery power, it is desirable that protection circuitry consume as little power as possible. Additionally, many battery applications are particularly cost sensitive. For such applications, it is important that the battery protection circuitry be relatively simple, compact, and easily manufactured.
It has been determined that the operating temperature of lithium-ion and similar batteries plays a significant role in efficient battery operation. It is desirable, for example, to charge such a battery only when the temperature is within certain ordinary limits, such as between about 0xc2x0 C. and about 35xc2x0 C. Also, battery life may be reduced if the battery is exposed to very high temperatures while being in a fully charged state. It is desirable to provide battery protection circuitry capable of addressing these operational concerns.
In accordance with the present invention, a battery protection circuit is disclosed that addresses several special considerations in the use of lithium-ion and similar batteries, such as temperature-dependent operation, undesirable exposure of the battery to moisture, and avoiding certain conditions that may shorten battery life.
The disclosed battery protection circuit includes a moisture detection circuit, a temperature sensing circuit, and a high-temperature battery discharge circuit. The moisture detection circuit includes a pair of conductive traces closely spaced on a substrate such that a resistive path is formed between the traces by moisture that forms on the substrate. The traces are connected between the positive battery terminal and a pull-down current source. When moisture forms on the substrate, a resistance is formed between the traces. The resulting pull-up current that flows between the traces causes a voltage change on one trace, which is detected by a circuit element such as a logic inverter. The output of the circuit element can be used to provide an indication to a user or to initiate other appropriate action.
The temperature sensing circuit includes a voltage reference circuit that generates a proportional-to-temperature voltage and temperature-independent voltage reference signals corresponding to various predetermined temperatures. A measuring circuit operates during a low-duty-cycle sampling interval to compare each temperature-dependent voltage to the proportional-to-temperature voltage and to store the result of each comparison until a subsequent temperature sampling interval. The measuring circuit includes multiplexing circuitry used to sequentially select each temperature-dependent voltage during the sampling interval, and provide the selected voltage to a comparator along with the proportional-to-temperature voltage. The output of the comparator is then selectively stored in a corresponding one of a set of storage devices such as flip/flops. The outputs of these storage devices can be used for a variety of operations that rely on information about the temperature of the battery.
The high-temperature discharge circuit connects a discharge load across the battery when the voltage of the battery is above a high voltage threshold and the temperature of the battery is above the high temperature threshold. When no external charger is active, the discharge current flowing through this load works to reduce battery voltage. By reducing battery voltage when the battery is exposed to high temperature, battery life can be prolonged.
Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows.